The present invention relates to the selective detection and identification of fuse failures in situations where the fuses being monitored are fixed to a rotating device such as a brushless exciter used with an electrical generator. Typically, brushless exciters have many fused circuits whose conductivity are important for the proper functioning of the exciter and corresponding generator.
Brushless exciters have been in development and use since 1950. Their basic function is to generate an alternating current and convert this alternating current to a direct current that is used to produce the rotating field for electrical generators. This is accomplished by rectification of the alternating current with multiple diode circuits which are rigidly mounted to the brushless exciter's rotor. For a six phase wye double-way system, at least six diodes are required but many more are normally used due to spare legs required, multiple-pole designs and very high current requirements. Where high current capabilities are needed, many diodes are connected in parallel to reduce the current flow in any individual diode.
To protect these diodes from degradation due to overcurrent applications and to protect the exciter's operation from the effects of a shorted diode, typical designs include fuses in series with the diodes. In applications that utilize many fuses in parallel the failure of a single fuse may not have a deleterious effect on the fuses that are connected in parallel with it. Although each of the remaining parallel fuses will be required to carry an increased current load, this overcurrent is usually within the design capability of the fuses. However a single fuse failure, while not adversely affecting the performance of the brushless exciter, cannot be ignored for long periods of time since the other fuses are working under a slight overcurrent condition and any subsequent fuse failure could have serious effects on the brushless exciter's ability to function properly.
Implicit in the above discussion is the fact that, if multiple fuse failures occur, the operator may want to shut the exciter down in an orderly fashion and replace the failed fuses. Furthermore, if an entire phase of fuses fails, an immediate emergency shutdown of the system will be required in order to prevent further damage to the system.
For these reasons it is important to be able to determine not only if any fuses have failed, but how many have failed and the specific locations in the circuit of the failed fuses. Since brushless exciter rotors usually rotate at either 1800 RPM or 3600 RPM, the linear velocity of its peripherally mounted diodes and fuses is very high, making an inspection by an operator impossible without the aid of specially designed equipment.
Attempts to aid the operator in this function have generally approached the problem in one of two ways. The first technique involves the use of some means to create a visible signal when a fuse fails. One such device is disclosed in Mann, et al U.S. Pat. No. 3,866,196 which utilizes a projectile that, upon a fuse failure, is allowed to extend radially from the surface of the exciter rotor and be observed through the use of stroboscopic light. The Mann device utilizes a retaining wire to hold the projectile in place. This wire is connected electrically in parallel with the fuse and, when the fuse fails, the current has to pass through the retaining wire, causing it to overheat, rupture and release the projectile.
The idea of using a current path parallel to the fuse to activate a visible signal upon a fuse failure is also disclosed in Lessmann U.S. Pat. No. 3,030,531 which, instead of rupturing a retaining wire, uses the parallel current to activate a lamp which can be observed with stroboscopic light. The Lessmann disclosure further anticipates the use of a transparent fuse, instead of a lamp, wherein the transparent fuse is burnt when the current passes through it and the burnt condition is readily visible with stroboscopic light.
In summary, this first of two techniques involves the utilization of some device connected in parallel with each fuse in such a way that very little current flows through the device when the fuse is operating properly but, when the fuse blows, the device must carry a portion of the current that normally would flow through the fuse and this added current activates a signal which can be viewed with stroboscopic light. These methods all require that the design of the brushless exciter rotor itself be modified to include the signal device and that the operator make a deliberate investigation of the device with the aid of stroboscopic light.
The second of the two general techniques to assist the operator in discovering failed fuses requires neither a rotor design modification nor a deliberate investigation by the operator. This method, as disclosed and claimed in a copending application of W. H. South, Ser. No. 197,732, filed Oct. 16, 1980, and assigned to the Westinghouse Electric Corporation, utilizes the magnetic field generating property of electric currents to enable the remote selective determination and identification of failed fuses. The South device operates on the principle that, if a fuse is conducting an electric current, a magnetic field will exist around it and will move with the fuse as the exciter rotor rotates. This magnetic field can be remotely sensed by various devices and the conducting status of each fuse can be determined. Since any specific diode and its associated fuse do not conduct current during the entire 360 degrees of rotation of the exciter rotor, the South disclosure further devises a means to selectively activate fixed magnetic field sensors that are disposed about the rotor at locations where the various fuses are, by design, expected to have current flowing through them. Further, electronic circuitry allows for automatic display, alarm and shutdown sequences to be incorporated within the South device.
This second type of method for detecting failed fuses has the significant advantages of not requiring a portion of the detection device to be part of the exciter rotor design and eliminating the need for the operator to make a deliberate inspection of the failed fuse signalling devices. It also has the important characteristic of being failsafe. Instead of responding to the absence of current flowing through the fuse it senses the presence of current flowing through it. This aspect becomes significant under two circumstances. First, if the diode itself fails in a way that creates an open circuit its associated fuse and detection device will not be subjected to a current and thus, if that current is a requirement for a failed fuse indication, the detection system will not properly indicate a malfunction. Second, if the signalling device itself fails, the absence of the signal will indicate a properly operating fuse under all conditions. Typical of this latter situation is a faulty electrical connection in the Mann device or a faulty lamp in the Lessmann device. The South device eliminates these two problems by sensing the proper flow of current when it is expected and therefore, if the sensing mechanisms fail, it would signal a fuse failure when none existed thus failing safely.
Certain brushless exciter designs do not lend themselves readily to the application of remote sensing as described in the South disclosure. The South invention utilizes devices that sense the existence of the magnetic field that surrounds a conductor through which current is flowing. However, where a brushless exciter design has other nearby conductors that carry current in the opposite direction at the same time the effect is the cancellation of the magnetic field. Even in situations where the field isn't totally cancelled, a signal may be so weakened so as to make the South device inefficient in operation.
The purpose of the present invention is to provide the primary advantage of automatic fuse failure detection as described in the South device while eliminating the problems associated with cancelled magnetic fields due to other nearby conductors carrying current in the opposite direction.